The hopper wrote:
I have a Canon 7D which has an APS-C camera sensor and gives a crop factor of 1.6. Some literature I read says that as a consequence, a standard lens will provide a perceived zoom. So for example, if I have a 100mm lens, the camera sensor will provide the equivalent to a 160mm lens (100x1.6 = 160 mm). Other literature just says that it just provides a reduced image from that which a full sensor would provide. In other words a reduced image.
I can't see how a standard lens will provide a perceived zoom. I think it will just in essence crop the picture that would be provided by a standard lens. A lens will just do what it is designed to do and cannot zoom beyond its normal range ... or have I got it wrong?
In short - help!!!
I have a Canon 7D which has an APS-C camera sensor... (
show quote)
A prime lens (or zoom at a particular focal setting) will have a certain
actual focal length F and a certain
angle-of-coverage A. This is what the lens sees. The longer the focal length, the smaller the angle-of-coverage.
Every lens mount has a fixed
flange-to-frame distance (FFD).
At a given FFD, a lens with angle-of-coverage A will project a circular image having
a diameter D (the
image diameter).
The square or rectangular image sensor must fit within D.
Usually, the four corners of the sensor will touch or almost touch the image circle
having diameter D. If the sensor is smaller, it is a "crop sensor" camera: the
angle-of-viewseen by the sensor will be smaller than the angle-of-coverage of the lens.
The term "crop sensor" is someone confusing, since the sensor isn't cropped: the image is.
The term is not used in optical engineering.
A given lens has a certain "resolving power": the abiliity to separatee fine detail.
The finest detail that can be separated is the
resolution of the lens, which may vary
from one part of the projected image to another (usually the resolution is lower
off-axis).
The image sensor also has a certain resolving power, which is limited by many factors
in its construction, but primarily by pixel density. The total number of photocells is
the total potential resolution of the sensor. If it is color sensor, then it take three photocells
to produce one color "pixel".
The resolving power of a camera cannot exceed that of its lens. In other words, the
sensor captures the projected image. However, for good quality lenses at middle
appertures, the resolution of the projected image far exceeds that of the image sensor.
(were you to inspect the projected image with a loupe, you would see far more detail
than the camera will capture).
Any area within the image circle that does not fall on the sensor represents wasted
lens resolution.
On the other hand, if any or all of the corners of the sensor extend beyond the image
circle it creates hard vignetting. This represents wasted image sensor resolution.
It's easy to see that square frames represent a more efficient use of the projected image
than rectangular frames.
The maximum number of resolved details that the lens and sensor can capture is the
total resolution of the camera. It cannot exceed the number of "megapixels", but may
be far less. The light arriving at the lens contains a certain amount of information,
but this is decreased three first by the lens and second by the sensor.
Total resolution is imporant because it is fixed at exposure and cannot be increased.
However, many things that are done to an image file will decrease resolution: cropping,
some digital filters, lossy compression, printing, etc.
Focal lengths of lenses for crop-sensor cameras are often expressed in "35 mm equivalents",
so that a "normal" lens is still "50 mm", not something shorter.
Frames smaller than 35 mm have been in use for spy cameras (e.g., Minox) since 1936.
The were only used for amateur photography after Kodak introduced 110 cartridge film
in 1972. 110 film (13 mm x 17 mm format) was never used by professionals or serious
amateurs.
Today "small is the new large". Loaves of bread, tubs of yogurt, jars of fruit keep
getting smaller. Pretty soon a dozen eggs will be 11. No force in nature is as powerful
as marketing--it trumps math and physics every time!
If a manufacturer can sell you a unicycle for the price of a bicycle, it wins. To that end,
it will tell you that uniycles are new, improved, high-technology, more advanced, more
natural (like walking!), way cool, very trendy, all the hipsters are buying them, etc. So
of course they cost more than a regular bicycle....
"Ever get the feeling you've been cheated?" --John Lydon